Power Armour with a basic VI interface. Isn’t autonomous and requires a “pilot” to control it.

B Series

Anthropoid (humanoid robot, often shortened to “‘poid”) linked to a central VI which relays commands to these bots. Nicknamed “Working Joes” by humans. A command given to one B Series can be carried out by another B Series if the central VI deems it more convenient. L-4DY is the MSS Eternia’s central computer, and thus acts as the central VI for its B Series ‘poids.

C Series

Life-like anthropoids which each have their own VI/AI. Completely independent and does not require a central VI like the B Series. C-UB3, the protagonist of the “Ark Pathfinder” chapter, is the first C Series ever built, having been built on the centuries old Marin-Niran pathfinder ship MSS Eternia (The ship which led the Cocoon to Arcria in the first place). He posseses a true AI, and was built not long after the Eternia arrived at Arcria in 5-AM.

They work like they do in Fable: you have to meet certain conditions to get into their realms. It can be anything from answering a riddle to equipping specific gear. To open all of these will require you to pay attention to the game’s lore, as some Demon Doors make references to this lore.